<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<style type="text/css">
#dialogoverlay{
	display: none;
	opacity: .8;
	position: fixed;
	top: 0px;
	left: 0px;
	background: #FFF;
	width: 100%;
	z-index: 10;
}
#dialogbox{
	display: none;
	position: fixed;
	background: #000;
	border-radius:7px; 
	width:550px;
	z-index: 10;
}
#dialogbox > div{ background:#FFF; margin:8px; }
#dialogbox > div > #dialogboxhead{ background: #666; font-size:19px; padding:10px; color:#CCC; }
#dialogbox > div > #dialogboxbody{ background:#333; padding:20px; color:#FFF; }
#dialogbox > div > #dialogboxfoot{ background: #666; padding:10px; text-align:right; }
</style>
<script>
function CustomAlert(){
	this.render = function(dialog){
		var winW = window.innerWidth;
	    var winH = window.innerHeight;
		var dialogoverlay = document.getElementById('dialogoverlay');
	    var dialogbox = document.getElementById('dialogbox');
		dialogoverlay.style.display = "block";
	    dialogoverlay.style.height = winH+"px";
		dialogbox.style.left = (winW/2) - (550 * .5)+"px";
	    dialogbox.style.top = "100px";
	    dialogbox.style.display = "block";
		document.getElementById('dialogboxhead').innerHTML = "Acknowledge This Message";
	    document.getElementById('dialogboxbody').innerHTML = dialog;
		document.getElementById('dialogboxfoot').innerHTML = '<button onclick="Alert.ok()">OK</button>';
	}
	this.ok = function(){
		document.getElementById('dialogbox').style.display = "none";
		document.getElementById('dialogoverlay').style.display = "none";
	}
}
var Alert = new CustomAlert();
</script>
</head>
<body>
<div id="dialogoverlay"></div>
<div id="dialogbox">
  <div>
    <div id="dialogboxhead"></div>
    <div id="dialogboxbody"></div>
    <div id="dialogboxfoot"></div>
  </div>
</div>
<h1>My web document content ...</h1>
<h2>My web document content ...</h2>
<button onclick="alert('You look very pretty today.')">Default Alert</button>
<button onclick="Alert.render('You look very pretty today.')">Custom Alert</button>
<button onclick="Alert.render('And you also smell very nice.')">Custom Alert 2</button>
<h3>My web document content ...</h3>
</body>
</html>